Late reply but maybe this will still help. I moved from a philosophy BA, where I studied a certain amount of philosophy of mind, but not really cognitive science (ethics was my strongest area), to a cognitive science master's. I'm totally pleased I did it, as cognitive science is incredibly interesting; there's certainly no academic field or career path I would prefer to have pursued in hindsight. I have been accepted onto a PhD in philosophy of cognitive science which I am planning to begin this autumn.
I don't think there is any need for your original degree to have been in the same discipline as the one you want to do a PhD in, if your master's covers the field in question. There may be some non-academic applications for your combination of degrees, although I'm not sure what they are! But do it because it's interesting!